This invention relates to coating compositions for forming mar and weather-resistant protective coatings on plastic substrates, typically polycarbonate resin substrates, and a method for the surface protection of plastic substrates.
As glazing substitutes, shatterproof or highly shatter resistant transparent materials have been widely utilized for these decades. For example, plastic substrates, especially polycarbonate resins have superior transparency, impact resistance and heat resistance and are currently used as structural members instead of glass in a variety of applications including building and vehicle windows and instrument covers.
The polycarbonate resins, however, are inferior to glass in surface properties such as mar resistance and weather resistance. It is desired to improve the surface properties of polycarbonate resin parts. Nowadays, polycarbonate resin parts for use as vehicle windows and acoustic barrier walls along highways are required to withstand more than 10 years of weathering.
Known means for improving the weather resistance of polycarbonate resin parts include the lamination of a weather resistant acrylic resin film on the surface of a polycarbonate resin substrate and the formation of a ultraviolet absorber-containing resin layer on the resin surface, for example, by co-extrusion.
For improving the mar resistance of polycarbonate resin parts, it is known to coat thermosetting resins such as polyorganosiloxanes and melamine resins and to coat photo-curable resins such as polyfunctional acrylic resins.
As to the manufacture of transparent articles having both weather resistance and mar resistance, JP-A 56-92059 and JP-A 1-149878 disclose ultraviolet-absorbing transparent substrates having a primer layer loaded with a large amount of UV absorber added and a protective coating of colloidal silica-containing polysiloxane paint overlying the primer layer. However, several problems arise with this approach. The addition of a large amount of UV absorber, especially to the primer layer can adversely affect the adhesion to the substrate and to the protective coating of colloidal silica-containing polysiloxane paint to be applied on the primer layer. During heat curing step, the UV absorber can volatilize off. On outdoor use over a long period of time, the UV absorber will gradually bleed out, causing whitening. From the mar resistance standpoint, it is impossible to add a large amount of UV absorber to the protective coating of colloidal silica-containing polysiloxane.
From these considerations, it was also attempted to fix a UV absorber by silyl modification. For example, JP-A57-21476 discloses alkylcarbamyl addition products of alkoxysilyl or alkanoylsilyl. This method, however, involves complex steps and is uneconomical.
It is also proposed to produce silyl-modified UV absorbers by reacting aromatic UV absorbers such as 2,4-dihydroxybenzophenone with silane compounds containing hydroxyl and epoxy groups. The reaction is effected between hydroxyl groups in the former and epoxy groups in the latter in the presence of tetramethylammonium chloride as disclosed in JP-A 58-10591 and JP-A 58-8766 or an aluminum chelate as disclosed in JP-B 3-62177.
Since these silyl-modified UV absorbers have alkoxysilyl groups which undergo condensation reaction during the heat curing step, the absorbers are advantageously fixed within the coating during the heat curing step so that the absorbers might not bleed out. However, these UV absorbers are less resistant to weathering, probably because the conjugated system of the UV absorber is altered by silyl modification. In general, the silyl-modified UV absorbers are added to the outermost protective coating layer. In the prior art, it has never been attempted to add the silyl-modified UV absorbers to primer or undercoating compositions.
An object of the invention is to provide a coating composition having improved long-term weather resistance and surface protection performance on a plastic substrate, and suited as an outermost coating on such a substrate. Another object is to provide an article having a coating of the coating composition on a substrate.
A further object is to provide an undercoating composition comprising a silyl-modified UV absorber and capable of forming a protective coating having improved mar and weather resistance. A still further object is to provide a method for protecting a surface of a plastic substrate using the undercoating composition.
We have found that significant improvements are made when a protective coating composition comprising (1) the reaction product of a hydroxyl group-containing benzophenone compound with a silane and/or a (partial) hydrolyzate thereof, preferably the reaction product obtained by reacting a compound of the following general formula (A) with an epoxy group-containing silane of the following general formula (B) in the presence of a catalyst, and/or a (partial) hydrolyzate thereof, (2) a silane of the following general formula (C) and/or a (partial) hydrolyzate thereof, and optionally and preferably, (3) a microparticulate inorganic oxide containing titanium, cerium or zinc, and capable of absorbing light with a wavelength of up to 400 nm, or a protective coating composition comprising a co-hydrolyzate of components (1) and (2), and optionally and preferably, component (3) is applied and cured onto plastic substrates, typically polycarbonate resins. The benzophenone organic UV absorber does not bleed out because of silyl modification or detract from mar resistance because of good compatibility with component (2). The benzophenone organic UV absorber containing at least three OH groups in a molecule maintains a UV absorbing capability substantially unchanged despite silyl modification. The use of benzophenone organic UV absorber in combination with the microparticulate inorganic oxide capable of absorbing light with a wavelength of up to 400 nm achieves the synergistic effect of effectively absorbing light in a wide UV region for significantly improving the weather resistance of plastic substrates, typically polycarbonate resins. 
In formula (A), X, which may be the same or different, is hydrogen or a hydroxyl group, and at least one X is a hydroxyl group.
R1aSiR2b(OR3)4-a-bxe2x80x83xe2x80x83(B)
In formula (B), R1 is an epoxy group-containing organic group, R2 is an alkyl group of 1 to 10 carbon atoms or aryl group, R3 is hydrogen or a substituted or unsubstituted monovalent hydrocarbon group of 1 to 10 carbon atoms which may contain an oxygen atom, xe2x80x9caxe2x80x9d is equal to 1 or 2, xe2x80x9cbxe2x80x9d is equal to 0 or 1, and the sum of a and b is equal to 1 or 2.
R4mSiR2n(OR3)4-m-nxe2x80x83xe2x80x83(C)
In formula (C), R4 is a substituted or unsubstituted monovalent hydrocarbon group of 1 to 10 carbon atoms, R2 is an alkyl group of 1 to 10 carbon atoms or aryl group, R3 is hydrogen or a substituted or unsubstituted monovalent hydrocarbon group of 1 to 10 carbon atoms which may contain an oxygen atom, m and n each are equal to 0, 1 or 2, and the sum of m and n is equal to 0, 1 or 2.
Seeking for an undercoating composition capable of improving the adhesion and weather resistance of molded parts of thermoplastic resins such as polycarbonate which are subsequently coated with an organopolysiloxane coating, we have found that when the reaction product of a hydroxyl group-containing benzophenone compound with a silane and/or a (partial) hydrolyzate thereof, preferably the reaction product obtained by reacting a compound of the general formula (A) with an epoxy group-containing silane of the general formula (B) and/or a (partial) hydrolyzate thereof is added to the undercoating composition, the reaction product is so compatible with other components in the undercoating composition owing to the effect of silyl groups that a large amount of the reaction product may be added without detracting from the adhesion of the undercoating composition to the substrate or the protective coating. The UV absorber is firmly fixed in the undercoat, eliminating a whitening phenomenon caused by bleeding-out with time. Since the undercoat contains a large amount of the UV absorber, a UV absorber which can adversely affect mar resistance need not be added to the organopolysiloxane coating or if added, only a small amount thereof is satisfactory.
Accordingly, a first embodiment of the invention provides a protective coating composition having improved weather resistance, comprising
(1) 0.1 to 50 parts by weight of the reaction product of a hydroxyl group-containing benzophenone compound with a silane and/or a (partial) hydrolyzate thereof, and
(2) 100 parts by weight of a silane compound of the following general formula (C):
R4mSiR2n(OR3)4-m-nxe2x80x83xe2x80x83(C)
wherein R4 is a substituted or unsubstituted monovalent hydrocarbon group of 1 to 10 carbon atoms, R2 is an alkyl group of 1 to 10 carbon atoms or aryl group, R3 is hydrogen or a substituted or unsubstituted monovalent hydrocarbon group of 1 to 10 carbon atoms which may contain an oxygen atom, m and n each are equal to 0, 1 or 2, and the sum of m and n is equal to 0, 1 or 2, and/or a (partial) hydrolyzate thereof.
A second embodiment of the invention provides a protective coating composition having improved weather resistance, comprising a co-hydrolyzate resulting from co-hydrolysis of (1) 0.1 to 50 parts by weight of the reaction product of a hydroxyl group-containing benzophenone compound with a silane and/or a (partial) hydrolyzate thereof, and (2) 100 parts by weight of a silane compound of the following general formula (C):
R4mSiR2n(OR3)4-m-nxe2x80x83xe2x80x83(C)
wherein R4, R2, R3, m and n are as defined above and/or a (partial) hydrolyzate thereof.
Also contemplated herein is an article comprising a substrate and a weather resistant protective coating formed on a surface of the substrate from the protective coating composition defined above.
A third embodiment of the invention provides an undercoating composition comprising the reaction product of a hydroxyl group-containing benzophenone compound with a silane and/or a (partial) hydrolyzate thereof.
A fourth embodiment of the invention provides an undercoating composition comprising
(1) 0.1 to 50 parts by weight of the reaction product and/or the (partial) hydrolyzate of the third embodiment, and
(5) 100 parts by weight of an organic copolymer comprising 0.1 to 50% by weight of an alkoxysilyl group-containing acrylic and/or vinyl monomer and 99.9 to 50% by weight of another monomer copolymerizable therewith.
A fifth embodiment of the invention provides an undercoating composition of the third or fourth embodiment further comprising (6) 0.1 to 50 parts by weight of a compound containing a nitrogen atom and an alkoxysilyl group in a molecule.
A sixth embodiment of the invention provides an undercoating composition of the foregoing embodiments further comprising (7) 0.1 to 10 parts by weight of a light stabilizer having at least one cyclic hindered amine structure in a molecule.
In a further aspect, the invention provides a method for protecting a plastic substrate, comprising the steps of:
(i) applying an organic solvent solution of the undercoating composition of the third or fourth embodiment onto a plastic substrate,
(ii) evaporating the organic solvent and curing the coating of the undercoating composition,
(iii) applying the protective coating composition of the first or second embodiment or a colloidal silica-containing organopolysiloxane composition onto the undercoat, said colloidal silica-containing organopolysiloxane composition comprising a hydrolyzate or co-hydrolyzate of a silane compound of the following general formula (Cxe2x80x2):
R5mSiR2n(OR3)4-m-nxe2x80x83xe2x80x83(Cxe2x80x2)
wherein R5 is a C1-10 alkyl group, aryl group, halogenated alkyl group, halogenated aryl group, alkenyl group or an organic group having an epoxy, (meth)acryloxy, mercapto, amino or cyano group, R2 is an alkyl group of 1 to 10 carbon atoms or aryl group, R3 is hydrogen or a substituted or unsubstituted monovalent hydrocarbon group of 1 to 10 carbon atoms which may contain an oxygen atom, m and n each are equal to 0, 1 or 2, and the sum of m and n is equal to 0, 1 or 2, and colloidal silica, and
(iv) heating the top coating for curing.
In the weather resistant protective coating composition according to the first aspect of the invention, a first component is the reaction product of a hydroxyl group-containing benzophenone compound with a silane and/or a (partial) hydrolyzate thereof. The silane is reacted with hydroxyl groups on the benzophenone compound. Any benzophenone compound having at least one hydroxyl group may be used although benzophenone compounds of the following general formula (A) are preferred. Any silane compound having a functional group capable of reacting with hydroxyl groups on the benzophenone compound may be used although epoxy group-containing silane compounds are preferred.
Accordingly, the preferred first component is the reaction product obtained by reacting a benzophenone compound of the following general formula (A) with an epoxy group-containing organoxysilane of the following general formula (B) in the presence of a catalyst, and/or a (partial) hydrolyzate thereof. Specifically the reaction product is obtained by reacting hydroxyl groups on the benzophenone UV absorber (A) with epoxy groups on the epoxy group-containing organoxysilane (B). 
In formula (A), X, which may be the same or different, is hydrogen or a hydroxyl group, and at least one X is a hydroxyl group.
R1aSiR2b(OR3)4-a-bxe2x80x83xe2x80x83(B)
In formula (B), R1 is an epoxy group-containing organic group, R2 is an alkyl group of 1 to 10 carbon atoms or aryl group, R3 is hydrogen or a substituted or unsubstituted monovalent hydrocarbon group of 1 to 10 carbon atoms which may contain an oxygen atom, xe2x80x9caxe2x80x9d is equal to 1 or 2, xe2x80x9cbxe2x80x9d is equal to 0 or 1, and the sum of a and b is equal to 1 or 2.
Examples of the benzophenone compound (A) which is one reactant from which component (1) is prepared are given below. 
Of these, 2,2xe2x80x2, 4,4xe2x80x2-tetrahydroxybenzophenone is especially preferred because of its UV absorbing capability.
On the other hand, the epoxy group-containing organoxysilane (B) is represented by
R1aSiR2b(OR3)4-a-bxe2x80x83xe2x80x83(B).
In formula (B), R2 is an alkyl group of 1 to 10 carbon atoms or aryl group, for example, methyl, ethyl, propyl, hexyl, decyl or phenyl.
R3 is hydrogen or a substituted or unsubstituted monovalent hydrocarbon group of 1 to 10 carbon atoms which may contain an oxygen atom. The monovalent hydrocarbon groups include alkyl, alkenyl, alkoxyalkyl, acyl and aryl groups, for example, methyl, ethyl, propyl, isopropyl, butyl, hexyl, phenyl, isopropenyl, methoxyethyl and acetyl.
The letter xe2x80x9caxe2x80x9d is equal to 1 or 2, xe2x80x9cbxe2x80x9d is equal to 0 or 1, and a+b is equal to 1 or 2.
R1 is an epoxy group-containing organic group, examples of which are given below. 
Illustrative examples of the epoxy group-containing silane (B) include xcex3-glycidoxypropyltrimethoxysilane, xcex3-glycidoxypropylmethyldimethoxysilane, xcex3-glycidoxypropyltriethoxysilane, xcex3-glycidoxypropylmethyldiethoxysilane, xcex2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, xcex2-(3,4-epoxycyclohexyl)ethylmethyldimethoxysilane, xcex2-(3,4-epoxycyclohexyl)ethyltriethoxysilane, and xcex2-(3,4-epoxycyclohexyl)ethylmethyldiethoxysilane. From the standpoints of solubility in protective coating compositions and reactivity with benzophenone compounds, the preferred silane compounds are xcex3-glycidoxypropyltrimethoxysilane, xcex2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, xcex3-glycidoxypropylmethyldimethoxysilane, and xcex2-(3,4-epoxycyclohexyl)ethylmethyldimethoxysilane.
In effecting the reaction, compounds (A) and (B) may be respectively used alone or in admixture of two or more.
The amount of compound (B) used is not critical although it is preferred to use 0.5 to 3 mol, more preferably 1.0 to 2.5 mol of compound (B) per mol of compound (A). The reaction product with less than 0.5 mol of compound (B) may become less soluble when added to a coating composition, resulting in reduced fixation to the substrate and poor weather resistance. With more than 3 mol of compound (B), the absolute amount of compound (A) participating in UV absorption may become small, leading to insufficient UV absorption.
The catalysts used in the reaction are preferably quaternary ammonium salts as disclosed in JP-A 58-10591. Exemplary quaternary ammonium salts include tetramethylammonium chloride, tetraethylammonium chloride, benzyltrimethylammonium chloride, and benzyltriethylammonium chloride. The amount of the catalyst added is not critical although it is preferred to use 0.005 to 10 parts, more preferably 0.01 to 5 parts by weight of the catalyst per 100 parts by weight of compounds (A) and (B) combined. Less than 0.005 part of the catalyst may require the reaction to continue for a longer time. More than 10 parts of the catalyst can adversely affect the stability of the coating composition to which component (1) is added.
The reaction is usually effected in the presence of the catalyst by heating compounds (A) and (B) at a temperature of 50 to 150xc2x0 C. for about 4 to 20 hours. The reaction may be effected in a solventless system or in a solvent in which both compounds (A) and (B) are dissolved. The use of a solvent is rather preferable for ease of reaction control and handling. Suitable solvents include toluene, xylene, ethyl acetate and butyl acetate.
According to the invention, a partial or complete hydrolyzate of the above-described reaction product may also be used. Hydrolysis of the reaction product is effected by adding water to a lower alcohol solution of the reaction product in the presence of an acid catalyst. Exemplary lower alcohols are methanol, ethanol, isopropanol and butanol. Solvents compatible with these alcohols include ketones such as acetone and acetylacetone, esters such as ethyl acetate and isobutyl acetate, and ethers such as propylene glycol monomethyl ether, dipropylene glycol monomethyl ether, and diisopropyl ether.
A second component in the coating composition of the invention is a silane compound of the following general formula (C) and/or a (partial) hydrolyzate thereof.
xe2x80x83R4mSiR2n(OR3)4-m-nxe2x80x83xe2x80x83(C)
In formula (C), R4 is a substituted or unsubstituted monovalent hydrocarbon group of 1 to 10 carbon atoms. Included are alkyl, aryl, halogenated alkyl, halogenated aryl, and alkenyl groups and substituted monovalent hydrocarbon groups in which some or all of the hydrogen atoms in the foregoing groups are replaced by (meth)acryloxy, mercapto, amino or cyano groups. Illustrative examples include alkyl groups such as methyl, ethyl, propyl, isopropyl, butyl, hexyl, decyl, and cyclohexyl; aryl groups such as phenyl and phenethyl; halogenated alkyl groups such as 3-chloropropyl, 3,3,3-trifluoropropyl, and 3,3,4,4,5,5,6,6,6-nonafluorohexyl; halogenated aryl groups such as p-chlorophenyl; alkenyl groups such as vinyl, allyl, 9-decenyl and p-vinylbenzyl; (meth)acryloxy group-containing organic groups such as xcex3-methacryloxypropyl and xcex3-acryloxypropyl; mercapto group-containing organic groups such as y-mercaptopropyl and p-mercaptomethylphenylethyl; amino group-containing organic groups such as xcex93-aminopropyl and (xcex2-aminoethyl)-xcex3-aminopropyl; and cyano group-containing organic groups such as xcex2-cyanoethyl. It is noted that R2 and R3 are as defined above in formula (B). The letters m and n each are equal to 0, 1 or 2, and m+n is equal to 0, 1 or 2. The silane compounds used herein function as an adhesive binder.
Illustrative examples of the silane compound satisfying the above conditions include
trialkoxy or triacyloxysilanes such as
methyltrimethoxysilane,
methyltriethoxysilane,
methyltris(2-methoxyethoxy)silane,
methyltriacetoxysilane,
methyltripropoxysilane,
methyltriisopropenoxysilane,
methyltributoxysilane,
ethyltrimethoxysilane,
ethyltriethoxysilane,
vinyltrimethoxysilane,
vinyltriethoxysilane,
vinyltriacetoxysilane,
vinyltris(2-methoxyethoxy)silane,
vinyltriisopropenoxysilane,
phenyltrimethoxysilane,
phenyltriethoxysilane,
phenyltriacetoxysilane,
xcex3-chloropropyltrimethoxysilane,
xcex3-chloropropyltriethoxysilane,
xcex3-chloropropyltripropoxysilane,
3,3,3-trifluoropropyltrimethoxysilane,
xcex3-methacryloxypropyltrimethoxysilane,
xcex3-acryloxypropyltrimethoxysilane,
xcex3-aminopropyltrimethoxysilane,
xcex3-aminopropyltriethoxysilane,
xcex3-mercaptopropyltrimethoxysilane,
xcex3-mercaptopropyltriethoxysilane,
N-(xcex2-aminoethyl)-xcex3-aminopropyltrimethoxysilane, and
xcex3-cyanoethyltrimethoxysilane;
dialkoxysilanes or diacyloxysilanes such as
dimethyldimethoxysilane,
dimethyldiethoxysilane,
dimethyldi(2-methoxyethoxy) silane,
dimethyldiacetoxysilane,
dimethyldipropoxysilane,
dimethyldiisopropenoxysilane,
dimethyldibutoxysilane,
vinylmethyldimethoxysilane,
vinylmethyldiethoxysilane,
vinylmethyldiacetoxysilane,
vinylmethyldi(2-methoxyethoxy)silane,
vinylmethyldiisopropenoxysilane,
phenylmethyldimethoxysilane,
phenylmethyldiethoxysilane,
phenylmethyldiacetoxysilane,
xcex3-propylmethyldimethoxysilane,
xcex3-propylmethyldiethoxysilane,
xcex3-propylmethyldipropoxysilane,
3,3,3-trifluoropropylmethyldimethoxysilane,
xcex3-methacryloxypropylmethyldimethoxysilane,
xcex3-acryloxypropylmethyldimethoxysilane,
xcex3-aminopropylmethyldimethoxysilane,
xcex3-aminopropylmethyldiethoxysilane,
xcex3-mercaptopropylmethyldimethoxysilane,
xcex3-mercaptopropylmethyldiethoxysilane,
xcex3-mercaptopropylmethyldiethoxysilane,
N-(xcex2-aminoethyl)-xcex3-aminopropylmethyldimethoxysilane, and
xcex2-cyanoethylmethyldimethoxysilane;
tetraalkoxysilanes such as methyl silicate, ethyl silicate, n-propyl silicate, n-butyl silicate, sec-butyl silicate, and t-butyl silicate. Partial or complete hydrolyzates of these silane compounds are also useful.
These silane compounds and/or (partial) hydrolyzates thereof may be used alone or in admixture of two or more.
The (partial) hydrolyzates of the above silane compounds are obtained, for example, by adding water to a lower alcohol solution of the silane compound in the presence of an acid catalyst. Exemplary lower alcohols are methanol, ethanol, isopropanol and butanol. Solvents compatible with these alcohols include ketones such as acetone and acetylacetone, esters such as ethyl acetate and isobutyl acetate, and ethers such as propylene glycol monomethyl ether, dipropylene glycol monomethyl ether, and diisopropyl ether.
In the protective coating composition of the invention, 0.1 to 50 parts, preferably 0.5 to 50 parts, and especially 1 to 30 parts by weight of the reaction product and/or its (partial) hydrolyzate as component (1) is blended with 100 parts by weight as solids of the silane compound and/or its (partial) hydrolyzate as component (2). Blending of more than 50 parts of component (1) is uneconomical whereas less than 0.1 part of component (1) fails to provide desired weather resistance.
In the protective coating composition of the invention, a microparticulate inorganic oxide is preferably blended as a third component. This component is also designated an inorganic UV absorber since it is a microparticulate inorganic oxide capable of absorbing detrimental light rays with a wavelength of up to 400 nm which can cause decomposition and degradation of organic compounds. Since oxides of titanium, cerium and zinc have an ability to absorb light rays of up to 400 nm in wavelength, the microparticulate inorganic oxide must contain at least one of titanium, cerium and zinc. If necessary, a metal oxide other than the above-described ones is added to the inorganic oxide particles in any desired manner for the purpose of stabilizing the particles or improving weather resistance, and as long as the light absorbing ability is not impaired. The manner of adding the other metal oxide includes simple addition, mechanical adsorption of the other metal oxide to the periphery of inorganic oxide particles, coating of inorganic oxide particles on their surface with a thin film of the other metal oxide, formation of mixed crystals by the sol-gel method, and doping of inorganic oxide particles with the other metal oxide in crystal form. When mixed crystals are formed, the content of titanium, cerium or zinc is preferably at least 50%, especially at least 60% by weight. Examples of the other metal include Si (silica), Al (alumina), Sn (tin oxide), Zr (zirconia), Sb (antimony oxide), Fe (iron oxide), and rare earth metals (rare earth metal oxides) though not limited thereto. Of these, Si, Al, Sn and Zr are preferred.
The inorganic oxide particles should preferably have a particle size of 1 to 300 mxcexc, more preferably 1 to 200 mxcexc. Particles with a size of greater than 300 mxcexc may adversely affect light transmission. Particles with a size of less than 1 mxcexc are inadequate since they are unstable and difficult to prepare. The inorganic oxide particles may be used in the form of powder, water dispersion or organic solvent dispersion.
In the protective coating composition of the invention, 0 to 100 parts, preferably 0.1 to 100 parts, more preferably 0.5 to 100 parts, and especially 1 to 80 parts by weight of the inorganic oxide particles (3) is blended with 100 parts by weight as solids of the silane compound and/or its (partial) hydrolyzate (2). Less than 0.1 part of component (3) is ineffective for further improving weather resistance and achieving the combined effect with component (1). Blending of more than 100 parts of component (3) may adversely affect film strength and film transparency and is uneconomical.
In the second embodiment of the invention, the protective coating composition contains a co-hydrolyzate resulting from co-hydrolysis of the reaction product and/or its (partial) hydrolyzate (1) and the silane compound and/or its (partial) hydrolyzate (2). This means that the reaction product and/or its (partial) hydrolyzate (1) is previously incorporated into the hydrolyzate of the silane compound (2). Also in this embodiment, hydrolysis is effected by adding water to a lower alcohol solution of the reaction product and/or its (partial) hydrolyzate (1) and the silane compound and/or its (partial) hydrolyzate (2) in the presence of an acid catalyst. Exemplary lower alcohols are methanol, ethanol, isopropanol and butanol.
In this embodiment, 0.1 to 50 parts, preferably 2 to 10 parts by weight of the reaction product and/or its (partial) hydrolyzate as component (1) is mixed with 100 parts by weight of the silane compound and/or its (partial) hydrolyzate as component (2). More than 50 parts of component (1) is uneconomical and causes gelation during reaction leading to non-uniformity whereas less than 0.1 part of component (1) fails to provide desired weather resistance.
Where the microparticulate inorganic oxide (3) is blended in the coating composition of the second embodiment, preferably 0 to 100 parts, more preferably 0.1 to 100 parts, most preferably 2 to 20 parts by weight of component (3) is blended per 100 parts by weight of the silane compound and/or its (partial) hydrolyzate (2).
In the protective coating composition of the invention, colloidal silica is preferably blended as a fourth component. Colloidal silica is blended in an amount of 1 to 200 parts, especially 10 to 150 parts by weight of per 100 parts by weight of component (2). One exemplary blending procedure is by mixing 20 to 90 parts by weight of the silane compound and/or its (partial) hydrolyzate (2) with 10 to 80 parts by weight as solids of a colloidal silica containing silica fines having a particle size of 1 to 100 nm to a total amount of 100 parts by weight. The mixture is diluted with alcohol, water or water-miscible solvent to a nonvolatile concentration of 15 to 20% by weight. The dilution is ripened at room temperature for about 3 to 5 days or at 40 to 60xc2x0 C. for about 10 to 15 hours. The term xe2x80x9ccolloidal silicaxe2x80x9d is a dispersion of silica fines in water or an alcohol such as methanol, ethanol, isobutanol or diacetone alcohol.
Also, upon the above-described hydrolysis, the colloidal silica may be added along with the acid catalyst.
To the protective coating composition, a buffer solution and a curing catalyst are preferably added so as to provide adequate abrasion resistance. Examples of the curing catalyst include dimethylamine, acetic ethanol amine, formic dimethylaniline, benzoic acid, tetraethylammonium salts, sodium acetate, sodium propionate, sodium formate, and trimethylammonium benzoyl acetate. An appropriate amount of the curing catalyst added is 0.01 to 1 part, especially 0.02 to 0.4 part by weight per 100 parts by weight as solids of the protective coating composition.
From the standpoint of insuring stability, the composition is preferably adjusted to pH 2 to 7, at which silanol groups remain stable, and especially pH 3 to 6. The buffer used for pH adjustment may be a combination of acidic and basic compounds, for example, a combination of acetic acid and sodium acetate and a combination of disodium hydrogen phosphate and citric acid.
In the protective coating composition, well-known additives commonly used in conventional coating compositions are blended if necessary.
The protective coating composition is useful in protecting surfaces of various articles, especially plastic articles. Specifically the coating composition is applied to an article substrate to form a protective coating thereon. The plastic article substrates to which the composition is applicable include those of polycarbonate, polystyrene, modified acrylic resins, urethane resins, thiourethane resins, polycondensates of halogenated bisphenol A and ethylene glycol, acrylic urethane resins, halogenated aryl group-containing acrylic resins, and sulfur-containing resins. The benefits become more outstanding when the coating composition is applied to transparent plastic substrates, and especially polycarbonate resins.
When the coating composition is applied to an article substrate to form a protective coating thereon, it is preferred that a primer layer intervenes between the substrate and the coating in order to enhance the adhesion to the substrate, especially plastic substrate. The primer for forming the primer layer is preferably based on a hydrolyzable silyl group-containing vinyl polymer.
The hydrolyzable silyl group-containing vinyl polymer used in the primer is derived by incorporating 0.1 to 50%, especially 0.5 to 20% by weight of a silane coupling agent into a vinyl monomer. The vinyl monomer used herein is selected from among alkyl methacrylates such as methyl methacrylate, butyl methacrylate and 2-ethylhexyl methacrylate; alkyl acrylates such as methyl acrylate, ethyl acrylate, and butyl acrylate; vinyl ethers such as glycidyl methacrylate, acrylamide, acrylonitrile, vinyl acetate, ethyl vinyl ether, butyl vinyl ether, and hexyl vinyl ether; styrene and ethylene glycol dimethacrylate, and mixtures thereof. The silane coupling agent is selected from among vinyltrimethoxysilane, vinyltriethoxysilane, vinyltris(xcex2-methoxyethoxy)silane, vinylmethyldimethoxysilane, vinylmethyldiethoxysilane, xcex3-glycidoxypropyltrimethoxysilane, xcex3-glycidoxypropylmethyldimethoxysilane, xcex3-methacryloxypropyltrimethoxysilane, xcex3-methacryloxypropylmethyldimethoxysilane, acryloxypropyltrimethoxysilane, acryloxypropylmethyldimethoxysilane, N-xcex2-(aminoethyl)-xcex3-aminopropyltrimethoxysilane, N-xcex2-(aminoethyl)-xcex3-aminopropyltriethoxysilane, N-xcex2-(aminoethyl)-xcex3-aminopropylmethyldimethoxysilane, and N-xcex2-(aminoethyl)-xcex3-aminopropylmethyldiethoxysilane, and mixtures thereof. More than 50% by weight of the silane coupling agent would invite a higher hardness, poor flexibility and economic disadvantage. With less than 0.1% by weight of the silane coupling agent, the primer layer would have insufficient substrate adhesion and hardness. The hydrolyzable silyl group-containing vinyl polymer is readily obtained by adding a radical polymerization initiator to a solution of the above-described monomer and silane coupling agent, followed by heating to effect reaction. The initiator is selected from peroxides such as dicumyl peroxide and benzoyl peroxide and azo compounds such as azobisisobutyronitrile.
A solvent is used in the primer. Useful solvents include diacetone alcohol, propylene glycol monomethyl ether, ethylene glycol monomethyl ether, propylene glycol monoethyl ether, ethylene glycol monoethyl ether, isobutyl alcohol, isopropyl alcohol, n-butyl alcohol, n-propyl alcohol, acetone, methyl ethyl ketone, methyl isobutyl ketone, acetyl acetone, ethyl acetate, butyl acetate, xylene, and toluene. On use, the primer composition is generally diluted with the solvent into a solution containing 5 to 10% by weight of the hydrolyzable silyl group-containing vinyl polymer.
To the primer, ultraviolet absorbers may be added. Preferred are organic UV absorbers which are fully compatible with the hydrolyzable silyl group-containing vinyl polymer. Derivatives of compounds having a hydroxybenzophenone, benzotriazole, cyanoacrylate or triazine main skeleton are especially preferred. Also acceptable are polymers such as vinyl polymers having such a UV absorber incorporated on a side chain. Exemplary UV absorbers are 2,4xe2x80x2-dihydroxybenzophenone, 2,2xe2x80x2, 4,4xe2x80x2-tetrahydroxybenzophenone, 2-hydroxy-4-methoxybenzophenone, 2-hydroxy-4-methoxybenzophenone-5-sulfonic acid, 2-hydroxy-4-n-octoxybenzophenone, 2-hydroxy-4-n-dodecyloxybenzophenone, 2-hydroxy-4-n-benzyloxybenzophenone, 2,2xe2x80x2-dihydroxy-4,4xe2x80x2-dimethoxybenzophenone, 2,2xe2x80x2-dihydroxy-4,4xe2x80x2-diethoxybenzophenone, 2,2xe2x80x2-dihydroxy-4,4xe2x80x2-dipropoxybenzophenone, 2,2xe2x80x2-dihydroxy-4,4xe2x80x2-dibutoxybenzophenone, 2,2xe2x80x2-dihydroxy-4-methoxy-4xe2x80x2-propoxybenzophenone, 2,2xe2x80x2-dihydroxy-4-methoxy-4xe2x80x2-butoxybenzophenone, 2,3,4-trihydroxybenzophenone, 2-(2-hydroxy-5-t-methylphenyl)benzotriazole, 2-(2-hydroxy-5-t-octylphenyl)benzotriazole, 2-(2-hydroxy-3,5-di-t-butylphenyl)benzotriazole, ethyl-2-cyano-3,3-diphenyl acrylate, 2-ethylhexyl-2-cyano-3,3-diphenyl acrylate, 2-(2-hydroxy-4-hexyloxyphenyl)-4,6-diphenyltriazine, 4-(2-acryloxyethyl)-2-hydroxybenzophenone polymer, and 2-(2xe2x80x2-hydroxy-5xe2x80x2-methacryloxyethylphenyl)-2H-benzotriazole polymer. Of these, 2,2xe2x80x2, 4,4xe2x80x2-tetrahydroxybenzophenone is most preferred from the standpoints of compatibility with the primer and volatility. These organic UV absorbers may be used in admixture of two or more.
In the primer, the reaction product and/or its (partial) hydrolyzate defined above as component (1) may account for 10 to 100% by weight of the organic UV absorber.
An appropriate amount of the organic UV absorber is 0.5 to 15 parts, especially 0.5 to 7 parts by weight per 100 parts by weight of the hydrolyzable silyl group-containing vinyl polymer. More than 15 parts of the organic UV absorber would precipitate out on the coating to aggravate an outer appearance, and reduce the stability of the hydrolyzable silyl group-containing vinyl polymer. With less than 0.5 parts of the organic UV absorber, the desired weather resistance would not be obtained.
To the primer, a light stabilizer having at least one cyclic hindered amine structure in a molecule may be added for improving weather resistance. The light stabilizer used herein should preferably be fully soluble in the solvent of the primer, compatible with the hydrolyzable silyl group-containing vinyl polymer, and low volatile.
An appropriate amount of the light stabilizer added is 0.03 to 2 parts by weight per 100 parts by weight of the hydrolyzable silyl group-containing vinyl polymer. More than 2 parts of the light stabilizer would detract from the adhesion of the primer layer.
Illustrative examples of the light stabilizer include 3-dodecyl-1-(2,2,6,6-tetramethyl-4-piperidinyl)pyrrolidine-2,5-dione, N-methyl-3-dodecyl-1-(2,2,6,6-tetramethyl-4-piperidinyl)pyrrolidine-2,5-dione, N-acetyl-3-dodecyl-1-(2,2,6,6-tetramethyl-4-piperidinyl)pyrrolidine-2,5-dione, bis(2,2,6,6-tetramethyl-4-piperidyl) sebacate, bis(1,2,2,6,6-pentamethyl-4-piperidyl) sebacate, tetrakis(2,2,6,6-tetramethyl-4-piperidyl) 1,2,3,4-butanetetracarboxylate, tetrakis(1,2,2,6,6-pentamethyl-4-piperidyl) 1,2,3,4-butanetetracarboxylate, a condensate of 1,2,3,4-butanetetracarboxylic acid with 2,2,6,6-tetramethylpiperidinol and tridecanol, 8-acetyl-3-dodecyl-7,7,9,9-tetramethyl-1,3,8-triazaspiro[4,5]decane-2,4-dione, a condensate of 1,2,3,4-butanetetracarboxylic acid with 1,2,6,6-tetramethyl-4-piperidinol and xcex2,xcex2,xcex2,xcex2xe2x80x2-tetramethyl-3,9-(2,4,8,10-tetraoxaspiro[5,5]undecane)diethanol, and a condensate of 1,2,3,4-butanetetracarboxylic acid with 2,2,6,6-tetramethyl-4-piperidinol and xcex2,xcex2,xcex2,xcex2xe2x80x2-tetramethyl-3,9-(2,4,8,10-tetraoxaspiro[5,5]undecane)diethanol. For the purpose of fixing the light stabilizer, there may be also used silyl-modified light stabilizers as disclosed in JP-B 61-56187, for example, 2,2,6,6-tetramethylpiperidino-4-propyltrimethoxysilane, 2,2,6,6-tetramethylpiperidino-4-propylmethyldimethoxysilane, 2,2,6,6-tetramethylpiperidino-4-propyltriethoxysilane, and 2,2,6,6-tetramethylpiperidino-4-propylmethyldiethoxysilane. These light stabilizers may be used in admixture of two or more.
Most preferably, the protective coating composition of the invention is used in forming the outermost layer. Any of well-known application methods such as spraying, dipping, curtain flow, and roll coating may be used. The coating generally has a thickness of about 1 to 10 xcexcm, preferably about 2 to 5 xcexcm in the dry state. A coating thinner than 1 xcexcm may accomplish less surface protection. A coating thicker than 10 xcexcm is likely to crack upon heat curing. In applying the primer, any of well-known methods may be used.
Heat curing conditions are not critical although heating at 100 to 130xc2x0 C. for about one hour is preferred.
Next, the undercoating composition according to the second aspect of the invention is described.
The undercoating composition of the invention essentially contains the reaction product of a hydroxyl group-containing benzophenone compound with a silane compound and/or a (partial) hydrolyzate thereof. Specifically the reaction product is obtained by reacting hydroxyl groups on the benzophenone compound with the silane compound. It is the same as the above-described component (1) in the protective coating composition.
In one preferred embodiment, the undercoating composition contains (1) the reaction product of a hydroxyl group-containing benzophenone compound with a silane compound and/or the (partial) hydrolyzate thereof set forth just above, and (5) an organic copolymer comprising an alkoxysilyl group-containing acrylic and/or vinyl monomer and another monomer copolymerizable therewith. Owing to the alkoxysilyl groups introduced, the undercoating composition is endowed with reactivity with the protective coating layer overlying the undercoat layer and improved in adhesion. Crosslinking of alkoxysilyl groups together improves heat resistance and imparts durability. Additionally, the copolymer is well compatible with the reaction product, i.e., silyl-modified benzophenone compound.
If the content of the alkoxysilyl group-containing monomer is less than 0.1% by weight, heat resistance and durability are not improved, and compatibility with the reaction product or silyl-modified benzophenone compound is aggravated. If the content of the alkoxysilyl group-containing monomer is more than 50% by weight, the copolymer would become too hard, losing adhesion. Therefore, the content of the alkoxysilyl group-containing acrylic and/or vinyl monomer is 0.1 to 50% by weight, preferably 2 to 30% by weight while the content of the copolymerizable monomer is 99.9 to 50%, preferably 98 to 70% by weight. The alkoxy moiety of the alkoxysilyl group should preferably have 1 to 4 carbon atoms, especially 1 to 3 carbon atoms.
Examples of the alkoxysilyl group-containing acrylic monomer include 3-methacryloxypropyltrimethoxysilane, 3-methacryloxypropyltriethoxysilane, 3-methacryloxypropylmethyldimethoxysilane, 3-methacryloxypropylmethyldiethoxysilane, 3-acryloxypropyltrimethoxysilane, 3-acryloxypropyltriethoxysilane, 3-acryloxypropylmethyldimethoxysilane, 3-acryloxypropylmethyldiethoxysilane, 3-methacryloxymethyltrimethoxysilane, 3-methacryloxymethyltriethoxysilane, 3-methacryloxymethylmethyldimethoxysilane, 3-methacryloxymethylmethyldiethoxysilane, 3-acryloxymethyltrimethoxysilane, 3-acryloxymethyltriethoxysilane, 3-acryloxymethylmethyldimethoxysilane, and 3-acryloxymethylmethyldiethoxysilane. Of these, 3-methacryloxypropyltrimethoxysilane and 3-methacryloxypropylmethyldimethoxysilane are preferred for ease of handling, crosslinked density and reactivity.
Examples of the alkoxysilyl group-containing vinyl monomer include vinyltrimethoxysilane, vinyltriethoxysilane, vinyltris(2-methoxyethoxy)silane, vinylmethyldimethoxysilane, vinylmethyldiethoxysilane, vinylmethylbis(2-methoxyethoxy)silane, 3-vinyloxypropyltrimethoxysilane, 3-vinyloxypropyltriethoxysilane, 3-vinyloxypropylmethyldimethoxysilane, and 3-vinyloxypropylmethyldiethoxysilane. Of these, vinyltrimethoxysilane, vinyltriethoxysilane, and 3-vinyloxypropyltrimethoxysilane are preferred for ease of handling and reactivity.
Examples of the other monomer copolymerizable with the alkoxysilane monomer include alkyl methacrylates such as methyl methacrylate, butyl methacrylate, and 2-ethylhexyl methacrylate; alkyl acrylates such as methyl acrylate, ethyl acrylate, and butyl acrylate; vinyl ethers such as glycidyl methacrylate, acrylamide, acrylonitrile, vinyl acetate, ethyl vinyl ether, butyl vinyl ether, and hexyl vinyl ether; styrene and ethylene glycol dimethacrylate; and methacrylic group-containing benzotriazoles serving as a UV absorber such as 2-(2xe2x80x2-hydroxy-5xe2x80x2-methacryloxyethylphenyl)-2H-benzotriazole. Since compounds having a group capable of reacting with the alkoxysilyl group, for example, 2-hydroxyethyl methacrylate can cause the undercoating composition to change with time, typically thickening or gelling, it is recommended to use a monomer free of a group capable of reacting with the alkoxysilyl group.
The organic copolymer constituting the main component of the undercoating composition according to the invention is a copolymer of the alkoxysilyl group-containing monomer with the other monomer copolymerizable therewith. The copolymer is readily obtained by adding a radical polymerization initiator to a solution of the monomers, followed by heating to effect reaction. The initiator is selected from peroxides such as dicumyl peroxide and benzoyl peroxide and azo compounds such as azobisisobutyronitrile.
If the organic copolymer is less than 10% by weight of the undercoating composition, the composition may become thermoplastic and less resistant to heat. If the organic copolymer exceeds 80% by weight of the undercoating composition, adhesion may become poor. Therefore, an appropriate amount of the organic copolymer is 10 to 80%, especially 20 to 80% by weight of the undercoating composition.
The organic copolymer (5) and the reaction product (1) are blended such that 0.1 to 50 parts, especially 2 to 50 parts by weight of the reaction product (1) is available per 100 parts by weight of the organic copolymer (5). More than 50 parts of the reaction product (1) is uneconomical whereas less than 0.1 part of the reaction product (1) fails to provide the desired weather resistance.
If the undercoating composition has a too low viscosity to apply and thus forms only a thin coating, an acrylic polymer may be added as a component capable of imparting flexibility without detracting from adhesion.
Such useful acrylic polymers include poly(alkyl methacrylates) and poly(alkyl acrylates) such as poly(methyl methacrylate), poly(butyl methacrylate), and poly(butyl acrylate), and copolymers thereof. These acrylic polymers are effective for imparting flexibility to the undercoating composition without detracting from adhesion. The amount of the acrylic polymer added is desirably limited to 30% by weight or less based on the entire undercoating composition since more than 30% by weight of the acrylic polymer can preclude the composition from heat curing.
In the undercoating composition, (6) a compound containing a nitrogen atom and an alkoxysilyl group in a molecule may be added for the purposes of assisting the composition in forming a satisfactory bond having water resistance, and fixing within the coating the silyl-modified benzophenone compound in component (1), the organic copolymer (5) and optional light stabilizer by crosslinking with alkoxysilyl groups therein. Preferably the compound (6) contains at least one nitrogen atom and at least two alkoxysilyl groups in a molecule.
For the purpose of causing the coating to be densely crosslinked through Michael addition reaction between (meth)acrylic groups in the organic copolymer and amino groups, the preferred compound (6) is one obtained by reacting an amino group-containing alkoxysilane, amide group-containing alkoxysilane or amino group-containing alkoxysilane with an epoxy group-containing alkoxysilane and a silylating agent and converting the reaction product into an amide. The more preferred compound is one obtained by reacting an amino group-containing alkoxysilane with an epoxy group-containing alkoxysilane and a silylating agent and amidating the reaction product.
Illustrative examples of the components used herein are described. Examples of the amino group-containing alkoxysilane include 3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, 3-aminopropylmethyldimethoxysilane, 3-aminopropylmethyldiethoxysilane, N-(2-aminoethyl)-3-aminopropyltrimethoxysilane, N-(2-aminoethyl)-3-aminopropyltriethoxysilane, N-(2-aminoethyl)-3-amino-propylmethyldimethoxysilane, N-(2-aminoethyl)-3-aminopropyl-methyldiethoxysilane, 3-(trimethoxysilylpropyl)-aminopropyltrimethoxysilane, 3-(triethoxysilylpropyl)-aminopropyltriethoxysilane, 2-(trimethoxysilylpropyl)-aminoethyl-3-aminopropyltrimethoxysilane, and 2-(triethoxysilylpropyl)aminoethyl-3-aminopropyltriethoxysilane.
Examples of the amide group-containing alkoxysilane include ureidopropyltrimethoxysilane, ureidopropyltriethoxysilane, ureidopropylmethyldimethoxysilane, and ureidopropylmethyldiethoxysilane.
The process of obtaining the amide compound by eacting an amino group-containing alkoxysilane with an epoxy group-containing alkoxysilane and a silylating agent and amidating the reaction product is described below. The amino group-containing alkoxysilane is as exemplified above although N-(2-aminoethyl)-3-aminopropyltrimethoxysilane and N-(2-aminoethyl)-3-aminopropylmethyldimethoxysilane are preferred for adhesion and operation. The epoxy group-containing alkoxysilane is exemplified by those of the above formula (B) although xcex3-glycidoxypropyltrimethoxysilane, xcex3-glycidoxypropylmethyldimethoxysilane, and xcex2-(3,4-epoxycyclohexyl)ethylmethyldimethoxysilane are preferred for reactivity and operation. Examples of the silylating agent include hexamethyldisilazane, N,Nxe2x80x2-bis(trimethylsilyl)-formamide and N,Nxe2x80x2-bis(trimethylsilyl)urea. When the amino group-containing alkoxysilane reacts with an epoxy group-containing alkoxysilane, the silylating agent serves to protect the OH groups generated by the reaction for preventing reaction between OH groups and alkoxysilyl groups, thereby precluding a change with time of the reaction product.
Reaction of the amino group-containing alkoxysilane with the epoxy group-containing alkoxysilane and the silylating agent may be effected by adding dropwise the epoxy group-containing alkoxysilane to a mixture of the amino group-containing alkoxysilane and the silylating agent and heating the mixture for reaction. Alternatively, the amino group-containing alkoxysilane is reacted with the epoxy group-containing alkoxysilane, and the silylating agent is added to the reaction product for further reaction.
In this reaction, the amino group-containing alkoxysilane and the epoxy group-containing alkoxysilane are preferably used in such amounts that the molar ratio of epoxy groups to amino groups may range from 0.3/1 to 1.2/1. If the molar ratio of epoxy/amino is less than 0.3, only a less number of alkoxy groups per molecule participate in crosslinking, leading to short cure, and the entire molecule is not spread, leading to a weak surface bond. If the molar ratio of epoxy/amino is more than 1.2, amino (xe2x95x90Nxe2x80x94H) groups which can be amidated during subsequent amidation step become few, exacerbating water-resistant bond.
The reaction product is then amidated. For amidation, the reaction product may be reacted with a carboxylic acid halide, acid anhydride or acid isopropenyl ester such as acetic chloride, acetic bromide, propionic chloride, acetic anhydride, isopropenyl acetate or benzoyl chloride.
In the undercoating composition, 0.1 to 50 parts, especially 0.5 to 20 parts by weight of the compound (6) is blended per 100 parts by weight of the organic copolymer (5). An excessive amount of the compound (6) results in an undercoat layer having a too high crosslink density, a high hardness, and rather poor adhesion.
In the undercoating composition, (7) a light stabilizer having at least one cyclic hindered amine structure in a molecule may be added for improving weather resistance. The light stabilizer used herein should preferably be fully soluble in the solvent of the undercoating composition, compatible with the organic copolymer, and low volatile. In the undercoating composition, 0.1 to 10 parts, especially 2.6 to 10 parts by weight of the light stabilizer (7) is blended per 100 parts by weight of the organic copolymer (5). More than 10 parts of the light stabilizer detracts from adhesion of a coating. The light stabilizers used herein are as previously exemplified.
In the undercoating composition, a conventional ultraviolet absorber which has not been silyl modified may be added insofar as no detrimental effect is exerted. Such UV absorbers are organic UV absorbers compatible with the organic copolymer. Derivatives of compounds having a hydroxybenzophenone, benzotriazole, cyanoacrylate or triazine main skeleton are especially preferred. Also acceptable are polymers such as vinyl polymers having such a UV absorber incorporated on a side chain. Illustrative examples are as previously exemplified.
On use, the undercoating composition is diluted with a solvent. Useful solvents include diacetone alcohol, propylene glycol monomethyl ether, ethylene glycol monomethyl ether, propylene glycol monoethyl ether, ethylene glycol monoethyl ether, isobutyl alcohol, isopropyl alcohol, n-butyl alcohol, n-propyl alcohol, acetone, methyl ethyl ketone, methyl isobutyl ketone, acetyl acetone, ethyl acetate, butyl acetate, xylene, and toluene. The undercoating composition is generally diluted with the solvent into a solution containing 5 to 10% by weight of the organic copolymer prior to use.
The undercoating composition is applied to a surface of a plastic substrate, typically a plastic film, which has been cleaned, whereupon the diluting solvent is evaporated off at room temperature or elevated temperature, leaving a dry undercoat of about 1 to 10 xcexcm, preferably about 2 to 5 xcexcm thick. The organic solvent dilution should preferably have a viscosity of about 5 to 30 centistokes. A dilution with a viscosity of less than 5 centistokes would be difficult to form a thick coat whereas a dilution with a viscosity of more than 30 centistokes would be difficult to handle and apply. To the composition, a fluorine or silicone surfactant may be added for leveling of the coating, and a crosslinking/curing catalyst may also be added for accelerating cure.
The cured coating of the undercoating composition is obtained by heating the wet coating at 80 to 200xc2x0 C.
By covering plastic substrates such as plastic films and sheets with the cured coat of the undercoating composition, the substrates are improved in initial adhesion, heat resistance, hot-water resistance, and weather resistance. Advantageously, a topcoat is formed on the undercoat, using the protective coating composition of the first aspect or a conventional colloidal silica-containing organopolysiloxane composition.
The colloidal silica-containing organopolysiloxane composition used herein contains an organopolysiloxane and colloidal silica. The organopolysiloxane is a hydrolyzate or co-hydrolyzate of a silane compound of the following general formula (Cxe2x80x2):
R5mSiR2n(OR3)4-m-nxe2x80x83xe2x80x83(Cxe2x80x2)
wherein R5 is a C1-10 alkyl group, aryl group, halogenated alkyl group, halogenated aryl group, alkenyl group or an organic group having an epoxy, (meth)acryloxy, mercapto, amino or cyano group, R2 and R3 are as defined above in formula (B), m and n each are equal to 0, 1 or 2, and m+n is equal to 0, 1 or 2. The colloidal silica is obtained by dispersing silica fines having a particle size of about 1 to 100 mxcexc in water or an alcohol such as methanol, ethanol, isobutanol or diacetone alcohol. To the hydrolyzate or co-hydrolyzate, 5 to 70% by weight of the colloidal silica is added.
In formula (Cxe2x80x2), R5 is a C1-10 alkyl group, aryl group, halogenated alkyl group, halogenated aryl group, alkenyl group or an organic group having an epoxy, (meth)acryloxy, mercapto, amino or cyano group. Examples of the group represented by R5 include alkyl groups such as methyl, ethyl, propyl, isopropyl, butyl, hexyl, decyl, and cyclohexyl; aryl groups such as phenyl and phenethyl; halogenated alkyl groups such as 3-chloropropyl, 3,3,3-trifluoropropyl, and 3,3,4,4,5,5,6,6,6-nonafluorohexyl; halogenated aryl groups such as p-chlorophenyl; alkenyl groups such as vinyl, allyl, 9-decenyl and p-vinylbenzyl; epoxy group-containing organic groups such as 3-glycidoxypropyl, xcex2-(3,4-epoxycyclohexyl)ethyl, and 9,10-epoxydecyl; (meth)acryloxy group-containing organic groups such as xcex3-methacryloxypropyl and xcex3-acryloxypropyl; mercapto group-containing organic groups such as xcex3-mercaptopropyl and p-mercaptomethylphenylethyl; amino group-containing organic groups such as xcex3-aminopropyl and (xcex2-aminoethyl)-xcex3-aminopropyl; and cyano group-containing organic groups such as xcex2-cyanoethyl. R2 and R3 are as defined above in formula (B). The letters m and n each are equal to 0, 1 or 2, and m+n is equal to 0, 1 or 2.
Illustrative examples of the silane compound satisfying the above conditions include the exemplary compounds described in conjunction with the general formula (C) as well as xcex3-glycidoxypropyltrimethoxysilane, xcex3-glycidoxypropylmethyldimethoxysilane, xcex3-glycidoxypropyltriethoxysilane, xcex3-glycidoxypropylmethyldiethoxysilane, xcex2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, xcex2-(3,4-epoxycyclohexyl)ethylmethyldimethoxysilane, xcex2-(3,4-epoxycyclohexyl)ethyltriethoxysilane, and xcex2-(3,4-epoxycyclohexyl)ethylmethyldiethoxysilane. These silane compounds may be used alone or in admixture of two or more.
(Co)hydrolysis of the silane compound(s) is effected by adding water to a lower alcohol solution of the silane compound(s) in the presence of an acid catalyst. Exemplary lower alcohols are methanol, ethanol, isopropanol and butanol. Solvents compatible with these alcohols include ketones such as acetone and acetylacetone, esters such as ethyl acetate and isobutyl acetate, and ethers such as propylene glycol monomethyl ether, dipropylene glycol monomethyl ether, and diisopropyl ether.
The (co)hydrolyzate or organopolysiloxane should preferably have a number average molecular weight of about 300 to 10,000, especially about 500 to 2,000.
In the organopolysiloxane composition, any of well-known additives commonly used in conventional coating compositions are blended if necessary.
For example, there may be added an effective amount of a microparticulate inorganic oxide capable of absorbing detrimental light rays with a wavelength of up to 400 nm which can cause decomposition and degradation of organic compounds, which has been described as an inorganic UV absorber or component (3) in conjunction with the protective coating composition of the first aspect of the invention.
Also, a catalytic amount of a curing catalyst as previously described is preferably added. An appropriate amount of the curing catalyst used is 0.01 to 1 part, especially 0.02 to 0.4 part by weight per 100 parts by weight as solids of the organopolysiloxane composition.
The organopolysiloxane composition is applied onto the undercoat of the undercoating composition on a plastic substrate and cured by heating, typically at a temperature of 50 to 140xc2x0 C. In this way, a top coat is formed on the plastic substrate to a high bond strength. The top coat of organopolysiloxane synergistically cooperates with the undercoat of the undercoating composition to accomplish high adhesion and abrasion resistance as well as excellent weather resistance and its stability due to tight fixation of the UV absorber in the undercoat. The top coat generally has a thickness of about 1 to 10 xcexcm though not critical.
The undercoating composition of the invention is applicable to various plastic materials including polycarbonate, polystyrene, modified acrylic resins, urethane resins, thiourethane resins, polycondensates of halogenated bisphenol A and ethylene glycol, acrylic urethane resins, halogenated aryl group-containing acrylic resins, and sulfur-containing resins. Outstanding benefits are obtained when the undercoating composition is applied to transparent polycarbonate resins.